The present invention relates to tuning systems for radio frequency devices such as radio transmitters and receivers and, more particularly, to improvements in such systems which are particularly adapted for use in a radio transceiver.
In conventional radio receivers, the tuning is often accomplished by a superheterodyne circuit; that is, one in which a mixer combines or "beats" an incoming radio-frequency signal with an internally-generated ("local oscillator") frequency to obtain a difference signal which is applied to the intermediate frequency (IF) circuitry of the receiver in a well-known manner. In some receivers, more than one mixer-local oscillator stage is employed and two intermediate frequencies are thus produced (e.g., 10.7 MHz and 455 KHz). The frequency of the local oscillator typically is made variable by a user-operable control to thus permit tuning of the receiver to various desired radio frequencies.
One particular type of local oscillator tuning system employs the well-known phase-locked-loop (PLL) technique in a tuning system which has sometimes been referred to as the "homodyne" or "synchrodyne" circuit. Although these circuits have been known and used for at least 35 years, the relatively high cost thereof had made them unfeasible for most commercial radio devices. More recently, however, integrated circuit (IC) technology has revitalized and refined the phase-locked-loop technique to the point where it has become quite attractive for use in modern radio frequency devices, particularly where it is desired to tune to a plurality of frequencies with a relatively high degree of precision.
In general, the phase-locked-loop is a feedback system composed of four basic elements: a phase detector or comparator, a low-pass filter, a voltage-controlled oscillator, and a feedback loop containing a frequency divider. In operation, a reference signal having a fixed frequency (often crystal-controlled for stability) is applied to one input of a phase detector and a second signal having a frequency relatively close to that of the reference signal is applied to the second input of the phase comparator. The phase comparator produces an output signal having a voltage that is proportional to the phase difference between the frequency of the reference signal and the frequency of the other signal. The output signal of the phase detector is filtered by a low-pass filter to obtain a DC error signal which is applied to the voltage-controlled oscillator to adjust its frequency. This frequency is applied, by means of the frequency divider, back to the second input of the phase comparator.
Thus, the frequency of the voltage controlled oscillator is changed until it reaches that frequency which, when divided by the divider in the feedback loop, equals the frequency of the reference signal, at which point it is locked in. In so-called frequency synthesizer circuits employing the PLL technique, the divider is made variable or programmable in a predetermined fashion so that a number of predetermined frequencies can be generated with considerable accuracy.
Such circuitry is well-known and reference may be had, for example, to the Apr. 11, 1968 issue of Electronic Design (pages 76-80); pages 52 and 53 of Floyd M. Gardner's book entitled "Phase Lock Techniques", John Wiley & Sons Inc., 1966; and U.S. Pat. No. 4,013,957 to Tojo. As shown in the 4,013,957 patent, for example, similar techniques also may be utilized in radio transceiver circuitry to produce the carrier frequency which is modulated by the information signal.
Although the recent developments in integrated circuit technology have greatly reduced the cost of phase-locked-loop circuits and have therefore made them quite popular in tuning systems for multi-channel radio receivers, transmitters, and transceivers, it is still desirable to simplify and further reduce the cost of such tuning systems. Moreover, it is desirable to reduce the amount of spurious signals, especially during transmission and without the need for special filter networks.
It is therefore an object of the invention to provide a new and improved tuning system for radio frequency devices.
It is another object of the invention to provide such a new and improved system which is highly adaptable to radio frequency receivers, transmitters, and transceivers to simplify the circuitry and reduce the costs thereof.
It is yet another object of the invention to provide such a tuning system which reduces spurious signal generation.